1. Technical Field of the Invention
This technology described herein relates generally to capacitive touch sensors and more particularly to circuits used to support capacitive touch sensors.
2. Description of Related Art
Various methods exist for detecting user interactions with a touch screen. Many of these utilize sensors whose electrical properties change when touched. Capacitive touch screens employ capacitive sensors for touch detection. Touching the screen in a specific area changes the amount of capacitance at that location which the system detects and after processing extracts as a user input.
Generally, capacitive touch sensors are capacitors with loosely confined electric fields purposely projected in ways that make their capacitance sensitive to touch. As a finger approaches a capacitive touch sensor, it intersects with the projected electric field lines causing the sensor capacitance to change. The amount of capacitance change depends on the quantity of field lines crossed. Therefore, the closer a finger is to a touch sensor the greater the changes in the sensor capacitance.
Usually, touch sensing is performed using either self or mutual capacitance sensors. In self-capacitance, the capacitive sensor is formed between a relatively small conductive layer and the much bigger sensor ground plane. In mutual capacitance, the capacitive sensor is formed between two conductive layers typically similar in size and neither assigned to any fixed voltages. Unlike self-capacitive sensors where only one electrode is accessible (i.e., available to make connection to other circuit elements), in mutual capacitive sensors both electrodes are accessible.
Often, touch sensing systems are designed to interface with either self or mutual capacitive sensors. Systems having the capability to sense both mutual and self-capacitance are less prominent as they often incur significant power and area overheads.
Another challenge in capacitive touch sensing is interference rejection. Capacitive touch sensors pick up both touch signals and additive interference. The frequency content of the two might overlap or can be too close to each other for direct filtering to be practical. Typically, capacitive touch sensing systems employ amplitude modulation to up-convert touch signals to a frequency band where they can be easily separated from interference using frequency selective filtering to achieve relatively error free touch detection.
FIG. 1 provides an example capacitive touch sensor circuit 100 with interference rejection. Touch sensor 103 receives a generated analog carrier signal (sine wave) to up-convert touch input signals generated when sensor 103 is touched. Specifically, waveform generator 101 generates a digital waveform which is then converted to analog by digital-to-analog converter (DAC) 102. The carrier frequency is selected such that the up-converted touch signal falls in a frequency range with low interference levels. The up-converted touch signals along with interference feed into trans-impedance amplifier 104 and subsequently pass through analog-to-digital converter (ADC) 105. The digital signals are demodulated at 106 using the digital carrier signal from waveform generator 101 and are fed into digital low pass filter (digital LPF) 107 for frequency selective filtering to remove interference. Data results (Data<N-1:0>) representing sensor capacitance values are output from the digital LPF and undergo further signal processing steps (not shown) to extract touch events. Although good interference rejection is achieved with such systems, dedicated digital-to-analog and analog-to-digital converters in addition to non-trivial digital signal processing blocks are required. These requirements increase the complexity of such systems making them less suitable for applications where low power and small area are necessary. Therefore, a need exists to provide a capacitive touch sensor having good interference rejection for low power and small area applications. Although there are many power and area efficient ways to sense capacitance, these schemes are usually inaccurate when interferers are present.
Disadvantages of conventional approaches will be evident to one skilled in the art when presented in the disclosure that follows.